Frequency modulation system



Patented Oct. 3 0, 1945 Raymond Wil'motte, Washington, D. i.

f This invention relates to frequencyjmodulation radio receivers, A and, particularly. to an imf` provedjcircuitfor `reception and detection of'a signal in the presenceY of anotherI of nearly equal intensity covering lparts'of a common frequency Vsignals before `reachingthe limiteriwhereby aysignal may be received satisfactorily lin" the piesence of an interfering signal of more nearly equal intensity than' is possible Vat present; lnthedrawingr Fig-'ure 1 shows in theform of ablock diagram a feed-back circuit. y 'f Figure 2 shows asimilar circuit 'as' in Figure 1, but in which the feed-back is super regenerative. Referring now'to Figure 1,v the conventional components cfa frequency modulationv receiver, which will be first described, include an antenna and ground2 and 4,respectively, connected to thev input of radio frequency amplifier' which is connected to a detector 8. A beat from oscillator I is combinedwith the output ofradio frequency amplifier 6` in detector 8 and passed to intermediate ffrequency amplifier I2 and thence through limiter I 4, discriminator 6, audio frequency amplifier I8 and finally to speaker 20.

- The ratio of the desiredy and undesired signal normally remain the same up to the input of the limiter circuit.4 The limiter circuit distorts the -wave -form and has the effect of increasing the ratio ofthe desired toundesired signal.

The inventionis concerned with the case where two or more signals, for instance, signals hereinafter termed A and B, impinge upon antenna 2, signal A, which is frequency modulated being stronger than signal B. In the normal receiver signal A would be interfered by signal B. In order to increase the eective ratio between the signals, an attenuator 22, which is variable, as indicated at 24, is connected across the output of limiter I4 ahead of discriminator IS and connected by a feed-back line 26 through circuit 28 55 Application December'26, 194,1,Y Serial`No. 424,51.?,1

` f `elclamis. ictzsol-zoi'f which maybe usedv to control the phase and amplitude. The output of circuit 28? which is connected back intointrme'diate frequency amplifier I2, is indicated at 30. *'YIhe phase? and amplitude are adjusted `so that regeneration is obtained without' oscillation. The `signals-"fed back are atthe same ratiol or intermediate free duencyA asthe" signal input to the lix'ni'terV but they cover-'Va different frequency fswing. Strictly speaking,` therefore, 'they are notu of the same frequency as the signals of` the input offthe limiter. The frequencylswingsat the input and output fof the limiter,"however,` are of fsuch a nature that the frequency modulationwill cor# respond to the same audio signals. For convene ience, the term basic frequency `will be used When two signals `have thef'i'samecarrierfrequency and the same type of modulationbut have a different frequency swing`,the signals will be said to have the same basic frequency When the weaker signal v is vfrequency *modulated `the ratio of the frequency swing of signalA, relative to that of signal B is greater at the output "ofthe limiter than at the input. Whenl the weaker signal is amplitude modulated, its degree ofinodulation is greatly reduced byifthe limiter and there isalso produced some phase modulation". The regeneration, thereforefwillbe greater for signal A than for signal `B and will producean effect equivalent to increasing the effective 'ratio of signal A to signal B at the input of :the limiter. With regeneration, therefore, Vthe ratio Vof sig'- nal A to signal `B will be `effectively greater at the input of the limiter than at the input of the receiver. The interferencecaused by signal B-wil therefore be decreased or eliminated. f" l The feed-back `may also be adjusted'as in a super regenerative receiver as shown in Figure'2 where the receiver oscillates periodically. AiBy;

this means theregenerative effect is substantially increased. Thesignals are'` received on antenna |02, amplified in |06, mixed with the'signal from oscillator ||0, in detector |08, amplified -inA the intermediate frequency amplier I2, passed `through a limiter I'I4', thence through a discriminator H6, an audio amplifier IIB and a loud speaker |20, a portion of the output of the limiter is taken by means of a potentiometer |22 with a tap |24 through a circuit |28 back to the intermediate frequency amplifier II2, at |30. The oscillator |3| feeding into the amplifier ||2 causes oscillation of the feed-back circuit periodcally in the manner well known in the art for super regeneration.

The invention described above is not limited to the specic disclosure, but should be given the full range of substitutions and equivalents within'the scope of the following claims. Specifically, for instance, the invention is applicable when the signals are of the same carrier frequency. It is also applicable with other forms of feed back than that specifically described above. It is also applicable when the type of modulation of the weaker signal is other than frequency modulation.

1. Tl'e method of receiving one of two signals occupying 4a common portion of two overlapping frequency bands, the stronger signal being fre' quency modulated, which comprises receiving said signals together, increasing the ratio of the stronger signal to the weaker 'signal and feeding back a portion of the signals vafter their ratio has been increased, the basic frequencies of the portion of the signals fed back and of the said two signals being thesame.

2. The method l of receiving -one of twosignals occupying a common portion :of two overlapping :Breguency bands, the strongersigna-l being frel quer-icymodulated, which comprises receiving said signals togethengincreasing the ratio kof the ampljtudeof the modulationof the stronger sig -n al relative tothe zother .and ,feeding back a pore tionfo the signals :after the-.ratio ,of theampli.- tuile of their modulation :has/been changed, the frequencies `of the portion-,of the :signals fed ,back and of :the said .two signals being the same. d

3,. In 'a frequency modulated radio .receiving system, means for separating rst and second signals xoccupying acommon portion of two .overlapping frequency bands, the rst signal being stronger and frequency modulated, comprising means for receiving the first and second signals, medifyingmeans whereby the ratio between vsaid first rand second signals in the .output of said modifying ,means is increased compared-With .the .ratio-*of sai'dfiirstand second signals at the Viniput of :said modifying fmeams, and circuit imeans for afeeding back l`a portion of -theou-tput of vsaid modifying lmeansinto the input of, said modifying said circuit means [beingysuch that at the Iinput of saidmodifying means the-basic freguencies -of v`the portion of the :signals fed back ,and .of ,the said firstJ and .second signals are the same. Y .j d A. In a yfreduency modulated :radio receiving system, means for separating rst ,and second signals occupying ,-a ycommon:portion @of two feuer Jappingfrequency bands, the rst signal being stronger ,and ffreguency modulated, ycomprising means for receiving the rst and second signals, modifying means whereby-liberati@ of the ampliftude'of fthe modulation of said first 1signalrelative to the amplitude of the-.modulation of `said sec sind signal in the output of said vllooliiying means 1 -is increased compared with-the ratio of ,the amplitude of .the `modulation -of said first signal relative to 'the amplltudefof the modul-action .of said second signalat the input of said back a portion of the output of said modifying means into the input of said modifying means, said circuit means being such that at the input of said modifying means the basic frequencies of the rportion of the signals fed back and of the said first and second signals are the same.

5. The combination claimed in claim 3, and means in said circuit means for modifying the phase and amplitude of the fed-back portion.

6J The combination claimed in claim 4, and

l-means in said `circuit means for modifying the I system, means for separating first and second isl simultaneous signals occupying a common portion of two overlapping frequency bands, the Viirst signalbeing 'stronger and frequency modulatedcomprising .ameans for receiving the first and second simultaneous signals, rst means for amplifying simultaneously said signals, second means having .an .input connected to the output oisaid first means, whereby the ratio of said first and second signalsisgreater in the Loutput ,of said second means thanin the input of said second means, `and circuit means Afor ,feeding lback a p0rtion of the output of said .Second means Vinto the input oisaid iirst mcanasaid circuit means :being `suchthat the inputof said first means, the basic ,frequenciesof the portion of .thesignal's fed yback and of the first and second signals are .the same.

8, In a frequency modulated radio receiving System, .means for separating first and ,second simultaneous signals occupying ka common portion of two overlapping frequency bands, the rStSienal being stronger and frequency modulated, comprising a means for receiving the first and second simultaneous signals, first means for amplifying ,simultaneously said signals., .second .means having an input connected .to the output of .said first means, whereby A the ratio .of the amplitude of the modulation-of said :first signal relative tothe amplitudecf modulation of said.

- second signal is greater in .the Output of said signals together, .modifying the `combined signals by increasing `,filierato of the stronger signal `to .the other signal, .reading .back :aportionof the modified signals at their basic :freguencm ,and reacting .the ted-.back portion ywith ,the signals prior to modioationso lthat at the point ofjfeed back, thelbasicfrequeucies of `the pori-,1011er the Signals lfed .back ,and of `the received signals yare the same.

RAYMOND M. 

